2552 was the penultimate year in the Human-Covenant War, during which the humans of the United Nations Space Command nearly saw utter defeat and extinction, though the tide turned, and with the help of the Sangheili, won the Human-Covenant War. The Covenant were at the peak of their power and reach in 2552, with major assaults on the human Inner Colonies and an invasion followed by a prolonged engagement on their home planet, Earth. During the battle at Installation 04, the galaxy found its first large outbreak of the Flood threat in over a hundred thousand years. Over two months later, an even larger outbreak occurred at Installation 05.
The war finally drew to a close at Installation 00, known more commonly as the Ark, with the humans and Sangheili attempting to assassinate the Prophet of Truth, before he could activate the remaining Halo installations. The alliance was successful in their mission against the Covenant.
- A team of ODST's (Including Kojo Agu and Taylor H. Miles) descend onto Ariel (a small science colony) in response to a distress call on the planet in Operation: FIRESIDE.
- Insurrectionists on Harmony disable the planet's primary communications relay and steal two drydocked freighters.
- Battle of Fumirole: Noble Team member Thom-A293 destroys a CCS-class battlecruiser but kills himself in the process, and Catherine-B320 loses her arm. She subsequently receives a robotic prosthetic.
- SPARTAN-B312 takes part in a counter-insurgency operation on Mamore. The Spartan's skills as a Sabre pilot are exhibited during the op, prompting Colonel Urban Holland to have him transferred to Noble Team.
After July 5
- Gabriel Thorne single-handedly saves his entire platoon in the Battle of Criterion.
- The Slipspace monitoring probes from Remote Scanning Station Archimedes at the edge of the Sigma Octanus system encounter the Covenant ships sent to attack Sigma Octanus IV.
- The UNSC Iroquois is tasked with patrolling the Sigma Octanus star system.
- The Battle of Sigma Octanus IV begins.
- The UNSC win a victory during the battle at Sigma Octanus IV. Commander Jacob Keyes, of the UNSC Iroquois, plays a critical role in this success when he devises the Keyes Loop maneuver. Keyes is subsequently promoted to Captain by Vice Admiral Michael Stanforth. Unknown to the UNSC, the Covenant place a tracker on the hull of the Iroquois. The importance of Reach to the UNSC is revealed to the Covenant when the Iroquois returns to that world.
- Vice Admiral Stanforth messages the director of the Office of Naval Intelligence, Admiral Margaret Parangosky, with information on the battle at Sigma Octanus IV, as well as the ongoing progress of Operation: BRAVADO.
- The Covenant deploy the Fleet of Valiant Prudence to sever Reach's out-system communications and search for Forerunner artifacts. This is the first stage of the Covenant's invasion of the planet.
- Noble Team finds and stops a pair of Insurrectionist SCI combatants attempting to hijack a supply transport.
- Fireteam 3 Charlie, along with other special warfare fireteams under command of Colonel Holland, are sent to investigate the areas around Longhorn Valley and Visegrád after contact with the comms relay in Visegrád is lost.
- Two months after the death of Thom-A293, SPARTAN-B312 joins Noble Team as Noble Six.
- Noble Team is sent by Colonel Holland on a mission to investigate the downed Visegrad Relay.
- Noble Team discovers the Covenant's presence on Reach for the first time at Visegrád Relay. Planetary directive WINTER CONTINGENCY is declared.
- Sword Base, a ONI research facility on Reach, comes under attack by an SDV-class heavy corvette. The UNSC Army with the help of Noble Team successfully repel the attack and the corvette is destroyed by orbital support.
- After discovering Reach, the Covenant move on to glass the other planets in the system, including Tribute, resulting in the prolonged Battle of Tribute.
- During the Battle of Tribute, SPARTAN-II Team Black is sent to Verge to destroy a Covenant Beacon supplying helium-3 to the ships laying siege on Tribute. The team successfully destroys the beacon.
- After the mission on Verge, Vice Admiral Alexander Reaves assigns Fireteam Black to Operation: BLOWBACK, a covert mission in Covenant-occupied space. Their ship, as well as a Covenant carrier, crash-land on an unknown moon, igniting the Battle of Line Installation 1-4.
- Black-One is captured by Forerunner Gatherers. Team Black and the Covenant forces on the moon enter into a reluctant alliance to rescue the Spartan.
- Fireteam Black encounters the Forerunner monitor 686 Ebullient Prism en route to rescuing Black-One. The monitor briefly causes rifts between the team to prevent them from rescuing their comrade. Black-One, however, escapes anyway, aided by Reff 'Talamee, and the rest of Team Black stop fighting amongst themselves.
- Team Black manages to enter the control room of Line Installation 1-4, eventually destroying 686 Ebullient Prism with the installation's main gun; they also succeed in destroying the entire Covenant fleet.
- Black-Box is created from a human brain donor to be the personal AI companion for Admiral Margaret Parangosky.
Early August (sometime from the 6th to the 12th)
- The UNSC Apocalypso crashes in lunar orbit, a slipspace anomaly knocking out communications within the Solar System for seven seconds.
- John-117 and Corporal Harland are debriefed by ONI at Camp Hathcock, Reach, on the events on Côte d'Azur during the Battle of Sigma Octanus IV.
- Battle at Szurdok Ridge: UNSC forces assault an amassing Covenant invasion force in the Viery Territory on Reach, managing to destroy two AA plasma cannons and a Covenant spire. However, the cloaked Covenant supercarrier Long Night of Solace reveals itself and destroys the frigate UNSC Grafton, which had been providing air support.
- 60% of the remainder of the UNSC fleet is recalled to Reach, destined to arrive within the next 48 hours.
- Operation: UPPER CUT is initiated, as Noble Team retakes the Sabre Program Launch/Research Range at Farkas Lake and joins the battle in orbit, eventually capturing the Covenant corvette Ardent Prayer and using it to deliver the UNSC Savannah's Shaw-Fujikawa Translight Engine, used as a makeshift bomb, to the Long Night of Solace . The Solace is destroyed at the cost of Jorge-052, who sacrifices himself to detonate the drive manually. Moments after the carrier's destruction, a large Covenant fleet enters in-system.
- Noble Six arrives in New Alexandria and helps evacuate civilians from the city. He also manages to severely damage one of the corvettes laying siege to the city by use of an AA missile battery. Noble Six eventually meets up with the rest of Noble Team, and the Covenant begin glassing the city. As Noble Team is on their way to a fallout shelter under the Olympic Tower, Catherine-B320 is killed by sniper fire.
- Dr. Halsey and Cortana choose the UNSC Pillar of Autumn, with Jacob Keyes as its commander, for a secret mission Halsey is planning.
- The remainder of Noble Team is evacuated from the destroyed New Alexandria via a Pelican.
- All available SPARTAN-IIs gather at the FLEETCOM Military Complex on Reach, where they learn of their next mission: to acquire a Covenant vessel, navigate to the Covenant home world, and return with captured Covenant leadership.
- John-117 receives a neural interface upgrade, MJOLNIR Mark V armor, and is introduced to Cortana.
- Noble Team is ordered to destroy Sword Base in a torch-and-burn op. During the operation, Noble Team makes their way inside Sword Base where they encounter Dr. Halsey, who asks them to protect her lab. Noble Team is then given direct orders to escort a fragment of Cortana holding crucial data from the Babd Catha Forerunner complex to the Pillar of Autumn. Halsey is then escorted back to CASTLE base by Jun-A266.
- Cortana has ONI Colonel James Ackerson illicitly transferred to the front. This is her revenge for his attempts to eliminate both her and SPARTAN-117.
- First Lieutenant Jake Chapman, along with most of Charlie Company is killed during the Fall of Reach.
- The Covenant, despite sustaining heavy losses, wins a crushing victory at the Fall of Reach. Many of the remaining SPARTAN-IIs are killed.
- The surviving SPARTAN-IIs on Reach are separated into two groups. One recovers Vice Admiral Whitcomb and goes into hiding at Camp Independence. The others are admitted into CASTLE Base by Dr. Halsey and receive parts of the MJOLNIR Mark V upgrade and new weapons.
- Dr. Halsey learns of Colonel Ackerson's mysterious plans that seem to involve "her" Spartans.
- The three surviving members of Noble Team arrive at Aszod; Carter-A259's Pelican becomes too badly damaged to continue carrying them all, so Noble Six and Emile-A239 deploy to the ground.
- Battle of Aszod: The Covenant forces throughout Aszod attempt to stop Noble Six and Emile from reaching the Pillar of Autumn, going so far as to deploy Scarabs against them. Carter sacrifices himself when he steers his damaged Pelican into a Scarab, destroying it. Emile and Six proceed to push towards the ship breaking yards and rendezvous with the scant UNSC forces still attempting to defend the facility. After clearing most of the facility, Six, Emile, and several Trooper squads meet up with Captain Jacob Keyes, successfully delivering the fragment of Cortana. However, Emile is killed by a Sangheili Zealot.
- SPARTAN-B312 destroys a CCS-class battlecruiser with a mass driver to allow the Pillar of Autumn to escape Reach. The Spartan is killed by Sangheili soon after. The Autumn jumps away from the system, with Cortana using Forerunner navigation data from the Sigma Octanus IV artifact as well as the Babd Catha Forerunner complex.
- The planet Coral is reported glassed by the Covenant.
- The Battle of New Jerusalem takes place. After a chain of events involving a group of ODSTs attempting to steal gold from a bank vault, the 26th Marine Expeditionary Force is nearly wiped out, leaving the Rookie as one of few survivors. Eventually, the UNSC forces cut their losses and fall back to Earth.
- Beta Gabriel is captured by a pack of Jiralhanae under the command of Chieftain Parabum. A majority of the sparse human population is either taken prisoner or killed, including ONI agent Connor Brien, who had been sent on a mission to study the Brutes. A recon force of ODSTs are sent in to neutralize the Brutes, and are successful in doing so.
- Lieutenant Wagner returns to Earth and reports on the evident destruction of Reach to the UNSC Security Council (Colonel James Ackerson, Fleet Admiral Terrence Hood, and Major General Nicolas Strauss) at HIGHCOM Facility Bravo-6 in Sydney, Australia. Hood determines that the location of Earth is compromised and decides that all ONI theatres of operation must be relocated. The Security Council also begins planning for the final defense of Earth.
- ONI launches an investigation of a SecNet conversation between Sergeants Joseph M. Poteet (Username: TRIGTECH) and Alasdair R. Schicker (Username: GLancer) to discover whether they might have accidentally leaked the location of Reach to the Covenant.
- The Battle of Installation 04 begins; the UNSC Pillar of Autumn arrives at Alpha Halo and is engaged by the Covenant Fleet of Particular Justice. John-117 is awoken from cryo sleep and tasked with preventing Cortana's capture by the Covenant. John-117 and Cortana escape the Pillar of Autumn aboard a Bumblebee escape pod, which later crashes on the ring.
- The UNSC Pillar of Autumn is forced to crash land on the ring. A significant portion of its crew survives to continue the fight on the ground.
- Captain Keyes and the others in his escape pod are captured by the Covenant. Only Keyes escapes summary execution, and is taken to the CCS-class Truth and Reconciliation for interrogation.
- A large group of ODSTs and survivors, under the command of Major Antonio Silva, (who assumed command of Autumn 's surviving crew after Captain Keyes' capture by the Covenant) establish Alpha Base, which serves as the main tactical firebase for the survivors and as a staging point for any future Covenant attacks.
- John-117 leads a Marine force to board the Truth and Reconciliation. They recover Keyes and escape aboard a Covenant dropship.
- Keyes orders John-117 to find the "Silent Cartographer", a map room that will reveal the location of the Control Room. Keyes leaves to lead an assault on a Covenant weapons cache.
- Having found the map room and the pertinent information, Cortana directs a Pelican to take them to the area where the ring's Control Room is. Upon reaching the Control Room, Cortana transfers to the Control Room's computer and directs SPARTAN-117 to stop Keyes from accessing the 'weapons cache', in reality a Forerunner Flood containment facility.
- John-117, without Cortana, enters the supposed weapons cache but is unable to locate Keyes. Instead, he discovers the Flood and is forced to retreat. He links up with marines and takes command of the surviving members of Keyes' team and together they fight their way to the extraction point. However, the Spartan is conscripted by 343 Guilty Spark and teleported away before he can be evacuated with the surviving members.
- Major Silva and the others under his command abandon Alpha Base after discovering a large Flood infestation beneath the base. They then learn that the Truth and Reconciliation has been abandoned and lead a strike force to capture the ship.
- The Flood succeed in capturing the Covenant dropship Brilliant Gift, and use it to crash-land onboard the Covenant ship Infinite Succor, in orbit above Installation 04, with the intent to spread itself to the rest of the Fleet of Particular Justice. This is avoided when Special Operations Commander Rtas 'Vadumee sets the Infinite Succor on a crash-course vector for the system's star, Soell. Rtas 'Vadumee is the only survivor.
- The SPARTAN-IV program enters development under the oversight of Admiral Margaret Parangosky, CINCONI. The project's existence is kept secret from both Dr. Catherine Halsey and Colonel James Ackerson, the respective creators of the SPARTAN-II and SPARTAN-III programs.
- The Monitor takes John-117 to the ring's Library to retrieve the Activation Index, a device needed to activate the ring's primary weapon. Having retrieved the Index, the Monitor teleports the Spartan back to the Control Room where an irate Cortana confiscates the Index and prevents the ring from firing. SPARTAN-117 and Cortana flee a now-hostile 343 Guilty Spark and Sentinels.
- To forestall the ring from being activated manually, John-117 disables the ring's three phase pulse generators. This is to buy time to destroy the ring by activating the auto-destruct on the Pillar of Autumn. Only Keyes' command neural interface can activate the ship's self-destruct, and SPARTAN-117 embarks once more to rescue the captain.
- Cortana teleports the Spartan to the now Flood-overrun Truth and Reconciliation where Jacob Keyes is being held. Alas, the pair arrives too late to save Keyes, who has been merged into a proto-Gravemind. John-117 retrieves the neural interface by punching in the former-captain's head. Cortana downloads the necessary codes, and the duo returns to the Pillar of Autumn on a captured Banshee.
- Major Silva's forces arrive at the Truth and Reconciliation and take over the ship through a combined surprise assault on the vessel's hangar bay and gravity lift. They also manage to capture the cruiser's prophet, the Prophet of Stewardship. After capturing the cruiser, Major Silva intends to fly the ship back to Earth in order to be recognized as the hero of the Battle of Installation 04; this acts a both a huge violation of the Cole Protocol and risks spreading the Flood. First Lieutenant Melissa McKay realizes this, so, in a snap-decision, she severs the primary optical fiber cable that linked controls from the bridge to engineering with a fragmentation grenade, thereby causing the cruiser to lose control and crash onto the surface of Halo.
- 343 Guilty Spark overrides the auto-destruct sequence, forcing John-117 to manually destabilize the ship's still-functioning fusion cores. John-117 and Cortana then escape the resulting explosion and the fragmentation of the ring aboard a Longsword.
- Staff Sergeant Johnson, Lieutenant Haverson, Warrant Officer Polaski, and Corporal Locklear also escape the destruction of the ring aboard a Pelican.
- In the course of the ring's destruction, a fragment of Installation 04 enters slipspace. This fragment later re-enters normal space in the vicinity of a red giant star.
- The Battle of Installation 04 ends. John-117 and Cortana rendezvous with Johnson's group in Installation 04's debris field. They also recover a set of cryo chambers, one which contains Linda-058.
- Skirmish over Threshold: As the Longsword and Pelican are not jump capable, the UNSC survivors assault and capture the Covenant flagship Ascendant Justice. They then set course for Reach.
- Jersey Morelli confronts his mother about having an affair.
- A time/date record anomaly continues from this point in the narrative, resolved above on Sep 7, 2552.
After the recovery of a Forerunner device on Reach, the members of an organized resistance comprised of Spartans from Reach and survivors of the Battle of Installation 04 are in some sort of time anomaly, causing them not to be in the "correct timeline". What is known is that the events taken place with the possession of the device take place after September 7.
- The time/date record anomaly of Sept 23, 2552 most likely rejoins the correct timeline.
- Dr. Halsey and members of Red Team discover an enigmatic shard in a Forerunner installation below CASTLE Base. They are besieged by Covenant forces who seek to possess the shard.
- Dr. Halsey initiates Operation: WHITE GLOVE, destroying CASTLE Base, thus preventing the Covenant forces above from capturing any UNSC or Forerunner technology. Halsey and members of Red Team are protected from the resulting detonation when they hide in the caves beneath the base.
- The remaining Spartans, Admiral Whitcomb, Dr. Halsey, Sgt. Johnson, Lieutenant Haverson, Warrant Officer Polaski, and Corporal Locklear jump to Slipspace on board the Covenant/UNSC hybrid ship UNSC Gettysburg-Ascendant Justice, and discover that the shard seems to have the ability to bend both space and time.
- The Gettysburg/Ascendant Justice arrives for refit and provisioning in the Eridanus system.
- Vice Admiral Whitcomb secures a meeting with rebel leader Governor Jacob Jiles, attempting to reason with him that the Covenant would most likely arrive to claim the Forerunner crystal. When several Covenant warships emerge from slipspace, Jiles agrees to help the UNSC forces.
- Dr. Halsey sedates Kelly-087, and absconds with her on Governor Jiles' personal ship, the Chiroptera-class Beatrice. Before she goes, she gives the Forerunner crystal to Corporal Locklear, with implicit instructions to destroy it.
- A large Covenant fleet enters the Eridanus system, forcing the hand of Admiral Whitcomb; he pulls out and retreats, as does the Gettysburg/Ascendant Justice, leaving the rebels to their doom.
- Governor Jiles is killed and the Eridanus rebels are destroyed during the Massacre at Eridanus Secundus. Eridanus Secundus itself is destroyed by the Covenant.
- UNSC forces aboard the Gettysburg/Ascendant Justice win a crucial victory at the Battle of the Unyielding Hierophant. SPARTAN-093, Lieutenant Haverson of ONI and Vice Admiral Whitcomb are killed. The survivors, now aboard the singular UNSC Gettysburg and with a captured Covenant slipspace drive, depart for Earth to warn of the impending Covenant invasion.
- ONI agent Jameson Locke compiles a target profile report concerning Supreme Commander Thel 'Vadamee, detailing 'Vadamee's personal history from the start of the Human-Covenant War to the glassing of Reach.
- An edict from the Office of the High Prophet of Truth orders that in the virtue of the Jiralhanae, all Sangheili posted with the Fleet of Profound Solitude, the Fleet of Tranquil Composure, and the Fleet of Inner Knowledge to be completely replaced by Jiralhanae. Each removed Sangheili to be assigned other duties. In celebration, all imprisoned or criminal Jiralhanae were released, rehabilitated, and pardoned.
- ODST Sarah Palmer, the future commander of the SPARTAN-IVs on UNSC Infinity, rescues Admiral Kovalic and a vital AI from a planet under Covenant attack. Wounded in action, Palmer is sent to Lockhart medical facility to recover.
- After escaping from the Battle of Tribute, the freighter Floral Express discovers Installation 03.
- Cortana details the Battle of Installation 04 to ONI.
- SPARTAN-117 is outfitted with the MJOLNIR Mk. VI aboard Cairo Station.
- Thel 'Vadamee, the Supreme Commander of the Fleet of Particular Justice, is deemed a heretic by the Covenant High Council and is sentenced to death.
- SPARTAN-117 and Staff Sergeant (now promoted to Sergeant Major) Johnson are awarded the Colonial Cross at a ceremony aboard Cairo Station. Captain Keyes is awarded posthumously and the decoration is entrusted to his daughter Miranda Keyes.
- The Battle of Earth begins.
- The High Prophet of Regret's fleet arrives at Earth, not expecting any human presence. The vastly outnumbered Covenant force attacks without seeking support from other Covenant forces.
- SPARTAN-117 foils a Covenant attempt to demolish Cairo Station and commandeers the Covenant bomb to destroy an assault carrier.
- The High Prophet of Regret's assault carrier, Solemn Penance, breaks through the orbital defenses and initiates a ground assault on New Mombasa and the surrounding area.
- SPARTAN-117 deploys to New Mombasa in an attempt to capture the High Prophet of Regret. Before this can happen, the assault carrier initiates a slipspace jump within the city, escaping Earth and severely damaging the city in the process. The UNSC In Amber Clad, with SPARTAN-117 aboard, successfully pursues the carrier through slipspace.
- Immediately after Regret's retreat, an additional Jiralhanae-led Covenant occupation force is inserted to New Mombasa, as a part of the Prophet of Truth's plan to eventually replace the Sangheili with the Brutes as the Covenant's primary warrior race. For the rest of the day, UNSC and Covenant forces battle for control of the city. Meanwhile, the Covenant fleet begins the process of uncovering the Ark portal.
- A squad of ODSTs led by Captain Veronica Dare and Gunnery Sergeant Edward Buck and consisting of Corporal Taylor "Dutch" Miles, Lance Corporal Kojo "Romeo" Agu, PFC Michael "Mickey" Crespo, and The Rookie, launch into New Mombasa from the UNSC Say My Name. However, the squad is scattered after the Prophet of Regret's carrier jumps to Slipspace.
- Gunnery Sergeant Buck lands at Tayari Plaza and assists Marine forces in combating the Covenant. He attempts to link up with Captain Dare, but she has gone missing, though he does link up with Lance Corporal Agu.
- Marine forces under the command of an unidentified colonel within New Mombasa start to regroup, managing to take back the Uplift Nature Reserve. However, the Mombasa Tether collapses, killing the Colonel and scattering the Marine forces into disarray.
- The ONI Alpha Site in New Mombasa is destroyed by the combined forces of the UNSC and the New Mombasa Police Department, led by Corporal Miles and and PFC Crespo, in order to prevent the Covenant from accessing it. During the attack, Police Commissioner Kinsler activates the fire extinguishing systems in the underground data facility, flooding its sublevels with argon. The argon kills Dr. Endesha, who had been on Sublevel 9. The resulting explosion also inadvertently awakens a large Drone hive in the tunnels below the base.
- The former Supreme Commander of Particular Justice is made the Arbiter by the High Prophets of Truth and Mercy.
- Gunnery Sergeant Buck and his squad regroup. They manage to capture a Covenant dropship and find a way out of New Mombasa through Kikowani Station, but abruptly turn around when they receive a distress call from Captain Dare.
- The Arbiter destroys a band of heretics led by Sesa 'Refumee and recovers 343 Guilty Spark during a raid on the Threshold gas mine.
- Sadie Endesha and Mike Branley manage to escape New Mombasa.
- The Battle of Mombasa ends with a Covenant victory.
- In the dawn, Covenant reinforcements sent by the Prophet of Truth continue arriving at Earth to uncover the Portal.
- Buck's ODST squad recovers a Huragok which has absorbed the Superintendent's Vergil subroutine, and escapes from the data facility beneath Alpha Site. Covenant forces attempt to capture Vergil from the squad as they make their way through the Waterfront Highway. The squad manages to hold out and eventually escapes New Mombasa as the city, along with its surroundings are glassed by the Covenant excavation fleet.
October 20 through November 2
- The Spartan-II Blue Team is deployed to defend Earth during the two weeks that follow the initial Covenant invasion. They participate in numerous combat missions on and around Earth, including Battle of Ross Island in the Antarctica and an underwater operation in the Yucatan Peninsula.
- The Office of Naval Intelligence sends an exploration-and-neutralization team to Gamma Halo.
- Skirmish on Mona Lisa: The UNSC Red Horse is sent to Installation 04's debris field in the Soell system to scout the area for survivors. They detect the prison ship Mona Lisa and deploy Marines into the ship. Soon after, it is revealed that the ship is infested by the Flood and was used by the Office of Naval Intelligence as a laboratory to test the parasite on prisoners. The infestation is contained as the Red Horse unleashes a Shiva-class nuclear missile on the Mona Lisa, destroying it.
- The Battle of Onyx begins.
- The standby signal rouses the Sentinels of Onyx, who destroy the UNSC base on the world.
- Team Gladius of the SPARTAN-III Gamma Company is totally wiped out by the Sentinels. Team Katana is forced into Zone 67, and Team Saber loses contact with Camp Currahee.
- Kurt-051, SCPO Franklin Mendez and Petty Officers Tom-B292 and Lucy-B091 begin to form a resistance against the Sentinels.
- The UNSC Agincourt, in high orbit above Onyx, is destroyed by Sentinels.
- Chief Mendez declares emergency code Bloody Arrow over the UNSC E-BAND.
The end of 2552 saw the bloodiest period of the Great War and the beginning of the end. The Prophet of Truth was killed in the later months along with a sizable Covenant armada, leaving the Covenant forces crippled and leaderless as well as the full destruction of the Gravemind and its Flood forces.
- The Battle of Installation 05 begins.
- The UNSC In Amber Clad arrives at Delta Halo, following the High Prophet of Regrets assault carrier. SPARTAN-117, aided by ODSTs and marines, pursue the High Prophet to a Forerunner temple and assassinate him. However, orbital bombardment from the escorting fleets of the newly arrived High Charity destroys the temple. SPARTAN-117 is thrown into a lake and captured by the Gravemind.
- The Great Schism begins.
- The Prophet of Conviction is killed by one of his shipmasters after learning that Conviction ordered the killings of the Sangheili councilors within the Fleet of Righteous Vigilance.
- The Arbiter is sent to Delta Halo to retrieve that installation's Index. The Arbiter is successful, but is then betrayed by Tartarus, who casts the Arbiter into the depths of the ringworld. The Arbiter is subsequently captured by Gravemind.
- Gravemind sends John-117 and the Arbiter to secure the Index and prevent the installation from being activated. SPARTAN-117 is teleported to High Charity while the Arbiter is teleported close to the Control Room. Both are aided by the chaos of the Covenant civil war.
- The Flood controlled In Amber Clad jumps inside High Charity and commences its infection. The Prophet of Mercy becomes infected by the Flood.
- SPARTAN-117 is unable to prevent the Index from leaving High Charity with Tartarus. Instead, on the advice of Cortana, the Spartan stows away aboard the Forerunner Dreadnought that is carrying the High Prophet of Truth to Earth. Cortana remains behind to ensure the destruction of Installation 05, should that become necessary.
- Cortana sends a slipspace message to Earth informing them that SPARTAN-117 is en route.
- The Arbiter reaches Installation 05's Control Room but is unable to prevent Tartarus from activating the ring. With the help of humans and the Sangheili, the Arbiter kills Tartarus. This allows Commander Keyes to physically remove the Index from the Core, temporarily interrupting the firing sequence. Due to the interruption, Installation 05 automatically sends a signal to all other Forerunner installations in the galaxy, activating and placing them in standby mode.
- The UNSC Dusk arrives at Delta Halo and witnesses the final stages of the Battle of Installation 05. She retreats to Earth via an anomalous slipspace wake caused by the Forerunner ship.
- Dr. Halsey and Kelly-087 arrive at Onyx and are trapped along with the rest of the human survivors. Halsey sends a distress signal piggybacked on Cortana's October 28 signal.
- The Battle of Havana is fought. William-043, Linda-058, and Fred-104 manage to stall the Covenant's forces in Havana, Cuba by destroying the Centennial Orbital Elevator. They receive new orders from Dr. Halsey to redeploy from Earth to Onyx, capture the Covenant destroyer Bloodied Spirit, and depart for Onyx.
- Voro 'Mantakree is promoted to Fleet Master by Imperial Admiral Xytan 'Jar Wattinree, and is given the mission to intercept the Spartans heading for Onyx and to retrieve the Forerunner technologies said to be there.
- Danforth Whitcomb's NOVA bomb detonates onboard the Covenant supercarrier Sublime Transcendence, in orbit above the Sangheili outpost world Joyous Exultation, destroying every ship in the vicinity, and also scorching a quarter of Joyous Exultation and completely vaporizing the nearby moon Malhiem.
- The Dusk arrives at Earth after mere hours. Upon arrival, she is re-assigned to Battlegroup Stalingrad. The group jump to Onyx via the Bloodied Spirit' slipspace wake.
- Blue Team arrives at Onyx aboard the captured Bloodied Spirit, and links up with Kurt-051, Kelly-087, Dr. Halsey, Chief Mendez, and the SPARTAN-III's. They proceed to push forward into Zone 67. The UNSC survivors are forced to retreat underground as a newly arrived Covenant fleet begins landing troops.
- Admiral Patterson's battle group arrives at Onyx to retrieve the survivors, but is unsuccessful due to the Covenant presence. The UNSC battlegroup is destroyed, with the exception of the Dusk, which avoids destruction by remaining stealthed.
- After an emergency mission redirect by the ONI AI Endless Summer, Kurt-051 and his forces proceed to destroy the Sentinel factory in Onyx's polar region and retrieve Team Katana, enclosed within slipspace field pods.
- Onyx is found to be an artificial structure built by the Forerunners to house a shield world. The human survivors hold off pursuing Covenant troops long enough to flee to the safety of the shield world.
- Fred-104 is promoted to Lieutenant, Junior Grade by Kurt.
- Kurt-051 dies after detonating 2 FENRIS warheads inside Onyx's core room antechamber.
- The Battle of Onyx ends. Sentinels annihilate the remaining Covenant forces as Onyx's artificial landmass disintegrates.
- Chol Von, a T'vaoan shipmistress, renamed her missionary ship Joyous Discovery as Paragon at some point of the Great Schism. During the Schism, she captured a Phantom, killing two Sangheili in the process.
- A Catalog unit, having been dormant for 100,000 years after the firing of the Halo Array, resumes its duties after receiving and denying a control authorization supposedly sent by the Librarian.
- The Prophet of Truth arrives in the Sol system aboard the Forerunner Dreadnought, and proceeds on course towards Earth at near-relativistic speeds.
November 8 through November 17
- The Battle of Mare Erythraeum takes place on Mars. Following the battle, Colonel James Ackerson, leader of the UNSC forces on Mare Erythraeum, is taken hostage and interrogated by the Jiralhanae Commander Lepidus onboard the Covenant cruiser Triumphant Declaration, who tells them of a Forerunner device linked to the Halo Array known as the Key of Osanalan, and that it can be found in Cleveland, Ohio. In reality, the Key of Osanalan is made up as a ploy by James Ackerson to prevent Cleveland from being glassed and to save the life of his brother Ruwan.
- Battle of Cleveland: Covenant forces, looking for the Key of Osanalan, invade the city of Cleveland, Ohio. After Ruwan learns the reason the Covenant are invading, he devises a plan with Sergeant Twyker. Injected with an organic tracking material, Ruwan confronts Lepidus at Chagrin Falls and tells him that he is the Key of Osanalan and is taken aboard the cruiser Harbinger of Piety, where the Minister of Inquisition vows to spare Earth in exchange for Ruwan's knowledge. Nassau Station then uses its Magnetic Accelerator Cannon to destroy the Harbinger of Piety.
- After the Key of Osanalan is revealed to be deception on part of Colonel Ackerson, Chieftain Lepidus executes Ackerson aboard Triumphant Declaration.
- Sergeant Major Johnson arrives back on Earth and gets clearance from Lord Hood to question Vergil about the artifact the High Prophet of Truth is digging up near Voi.
- The Forerunner Dreadnought enters Earth's atmosphere. On-board is Master Chief Petty Officer John-117, who ejects from the ship, using a hatch as a heat shield upon re-entry. Meanwhile, Battle of Sector Six takes place at Pawa, Afghanistan. There, numerous Marines of the 2nd ODST Battalion track SPARTAN-117's descent. SPARTAN-117 eventually lands in the East African jungle near Mount Kilimanjaro, going unconscious for hours.
- The Prophet of Truth's Dreadnought lands in the middle of the Portal near Voi, Kenya. During the last few weeks, Truth's fleet of twenty-odd battlecruisers and two assault carriers have almost entirely uncovered the Portal and occupied the entire region surrounding it.
- UNSC forces recover John-117 from a jungle near Mount Kilimanjaro. In the underground facility of Crow's Nest, a plan is formulated to launch an aerial strike on Covenant anti-air defenses in the town of Voi in order to prevent the Prophet of Truth from activating the Forerunner artifact. After repelling a Covenant assault in Crow's Nest and destroying the base in the process, a UNSC strike force led by John-117 retakes the Covenant-held Tsavo Highway.
- Battle of Voi: John-117 and a Marine strike force enter Voi and gradually push back the city's Covenant occupiers, destroying the Covenant anti-air defenses in the city. An aerial strike force consisting of two frigates and numerous Longsword fighters assaults the Forerunner Dreadnought resting over the Portal artifact, but are unsuccessful in damaging the ship. Truth activates the portal and retreats through it to Installation 00 along with his forces. At the same moment, a Flood-controlled ship slips in, crashing in Voi.
- UNSC forces make their way toward the crashed ship in order to overload its reactors and contain the Flood outbreak, just as the Sangheili Fleet of Retribution arrives and drops in reinforcements. After recovering a message by Cortana from the Flood ship, John-117 and the remaining human and Sangheili forces retreat from Voi, as the Fleet of Retribution glasses the city and its surroundings in order to contain the Flood.
- UNSC and Sangheili leaders gather aboard the Sangheili assault carrier Shadow of Intent to hear Cortana's message, which reveals there is a way to stop the Flood on the other side of the Portal. Despite Lord Hood's urging, Commander Keyes, along with the Master Chief and the Sangheili, enter the Portal with the Fleet of Retribution and two UNSC frigates.
- At 1545 hours, High Charity arrives at Mars and makes a slipspace jump to the lesser Ark.
- The Battle of Installation 00: The joint Sangheili-human force arrives at Installation 00 and immediately engages Truth's forces while the UNSC Forward Unto Dawn deploys ground forces to locate Truth. Despite being outnumbered three to one in space, the separatist fleet makes short work of the Covenant space forces. After locating Truth, the separatist-UNSC alliance launches an assault on the Ark's Citadel where Truth intends to activate the Halo Array. At the same time the former Covenant city ship High Charity arrives from slipspace and crashes on the Ark. Debris spread from the city-ship helps spread a new Flood infestation on the Ark.
- Miranda Keyes is killed when she attempts to rescue Johnson from Truth. With Truth moments away from activating the Halo array, John-117 and the Arbiter temporarily ally themselves with the Flood and storm the Citadel. They shut down the Halo array, and the Arbiter executes Truth, finally ending the Human-Covenant War. The Flood immediately turn against John-117 and 'Vadam, but the pair escape. Soon after, it is discovered that the Ark's foundry has created a replacement for the destroyed Installation 04. Realizing that the ring could be used to fire a tactical pulse which would eliminate the Flood infestation, John-117 decides to light it. In order to do so, he enters the ruins of High Charity and recovers Cortana, who still possesses the Activation Index from the original Installation 04. High Charity is destroyed in the process.
- With the Index secure, John-117, Cortana, the Arbiter, and Sergeant Major Johnson travel to the replacement Installation 04, while the rest of the UNSC and Sangheili forces return to Earth on the Shadow of Intent. In the control room, Johnson is killed as 343 Guilty Spark attempts to prevent him from activating the ring prematurely, as doing so would destroy the ring. John-117 destroys the monitor and begins the firing sequence for the Halo ring. SPARTAN-117 and the Arbiter then escape the control room by taking Johnson's Warthog to the Forward Unto Dawn, which is holding position nearby.
- Due to the unstable, incomplete nature of the ring as it fires, the slipspace portal collapses as the Dawn enters it, causing it to be sheared in two. Cortana and SPARTAN-117 are in the hangar area at the time of the separation, and are thus left in the aft section of the ship, which falls out of Slipstream space at an unknown location, eventually drifting towards a Forerunner shield world, Requiem.
- Arbiter Thel 'Vadam returns to Earth using the forward half of the Dawn, crashing into the Indian Ocean.
- The Shadow of Intent is repaired and refitted at a mobile shipyard.
- Codename: SURGEON filed a report to Codename: USUAL SUSPECTS entitled "Historical/Psychological Analysis of Cole, Preston J." The report was filed from UNSC Point of No Return in synchronous orbit on the far side of Luna.
- Operation: FIRESIDE
- Battle of Sigma Octanus IV
- Battle of Criterion
- Battle of Tribute
- Raid on Verge
- Battle of Line Installation 1-4
- Fall of Reach
- Battle of Installation 04
- Massacre at Eridanus Secundus
- Operation: FIRST STRIKE
- Battle of Earth
- Battle of the Gas Mine
- Battle of Installation 05
- Battle of Onyx
- Battle of Mare Erythraeum
- Battle of Installation 00
The Great Schism
Quarantine of the Flood
- All five Halo games produced by Bungie take place in the latter half of this year, while those produced by other studios take place before (Halo Wars) or after 2552 (the Reclaimer Saga and Spartan Assault) with the exception of Halo: Spartan Strike.
- The numerical value of the year is one of Bungie's many references to the number seven. Specifically, adding the numerals results in 14, which has twice the value of 7. Adding the two halves of the year separately (2+5 and 5+2) results in a pair of sevens.
- ^ Halo 2, campaign level Cairo Station
- ^ Halo: Ghosts of Onyx
- ^ Halo 3, campaign level The Storm
- ^ Halo: Combat Evolved, campaign level 343 Guilty Spark
- ^ Halo: Combat Evolved, campaign level The Library
- ^ Halo 2, campaign level Sacred Icon
- ^ Halo 3, campaign level The Ark
- ^ Halo 3, level The Covenant
- ^ Halo: Helljumper Issue 1 page 3 "UNSC Operation 2552-JA-8 Codename: Fireside"
- ^ a b c Halo: Reach, campaign level Noble Actual
- ^ Halo.xbox.com: Data Drop Two
- ^ Halo: Evolutions - Essential Tales of the Halo Universe - Blunt Instruments
- ^ a b Halo: Reach, campaign level Long Night of Solace
- ^ a b Halo: Reach, campaign level New Alexandria
- ^ Halo: Evolutions - Essential Tales of the Halo Universe - Dirt
- ^ Bungie.net: Halo 3: ODST Field Guide
- ^ Halo: The Fall of Reach (2010) - Adjunct
- ^ Halo: First Strike, page 104
- ^ Halo: The Flood, page 1
- ^ Halo Waypoint - Spartans
- ^ Halo Waypoint - Universe: Alpha Shard
- ^ Halo: First Strike
- ^ Halo 2 Anniversary, Terminals
- ^ Halo Wars: Halo Timeline
- ^ Halo: Initiation, Issue 1
- ^ a b c d Eleventh Hour reports
- ^ Halo: Evolutions - Essential Tales of the Halo Universe - "The Mona Lisa"
- ^ Halo Waypoint: Catalog Interaction (post 2976443)
- ^ Halo: Mortal Dictata, page 141
- ^ Halo: Mortal Dictata, page 73
- ^ Halo Waypoint: Catalog Interaction
- ^ a b Bungie.net: Bungie Weekly Update: 01/23/09
- ^ Halo Waypoint - Query: Catalog
- ^ Halo Waypoint, Hero-Fortitude
- ^ a b Waypoint: Glasslands complaints... forum thread
- ^ Halo Waypoint: Catalog Interaction - Page 14
- ^ Halo: Evolutions - The Impossible Life and the Possible Death of Preston J. Cole, pages 413-487